In medicine there is a need to frequently, and possibly, continuously take blood pressure. Novel devices have been created In recent times for this purpose, The method devised by Panaz contributed an essential novelty (Digest of the 10th International Conference on Medical and Biological Engineering 1973 Dresden), wherein light is shone through a finger and the registered flow Is kept constant by a booster control.
This photoplethysmographic method was taken up by several others also (Yamakoshi, Wesseling, TNO). EP 537 383 (TNO) Apr. 21, 1993 (21.04.93) discloses an inflatable finger cuff used for non-invasive continuous monitoring of blood pressure. The inflatable cylindrical space is connected pneumatically to a fluid source. An infrared light source and a detector are positioned on both sides of the finger inside the fixed cylinder. A valve is provided for filling the cylinder with gas. Electrical cables for the infrared light source and the detector are threaded through. U.S. Pat. No. 4,510,940 A (WESSELING) Apr. 16, 1985 (16.04.85) and U.S. Pat. No. 4,539,997 A (WESSELING) Sep. 10, 1985 (10.09.85) disclose a device for continuous non-invasive measuring of blood pressure. A fluid-filled cuff, a light source, a light detector and a differential pressure booster are provided. U.S. Pat. No. 4,406,289 A (WESSELING) Sep. 27, 1983 (27.09.83) also discloses such a device according to the prior art.
The cited documents all show prior art only, especially so when it is considered that features essential to the invention are missing in the main claim.
A major problem of these methods is on the one hand in the cuffs being used which have to be placed very precisely, are very interference-prone and not very durable, and on the other hand is with the proportional valves used which are very expensive to manufacture (U.S. Pat. No. 4,406,289) and also in the calibrating of the device which can very precisely indicate the relative fluctuations in blood pressure, wherein absolute measuring however deviates considerably from the actual intra-arterial values. Usually, with the proportional valves used to date either a) a toggle flapper is used, which can be moved alternately in one or the other direction by an electromagnet, or b) an electromagnetic shaker is used. With both these proportional valves there is a constant gas flow through the pressure chamber, as there is a part of the valve always open. Either the outlet opening is released into the open, or the inlet opening is released by the gas supply. There is no position of the valves, in which both inlet and outlet opening are simultaneously closed.
This results in very high gas consumption, of little relevance in fixed apparatus, but clearly significant in the case of portable units. A further drawback is the use of pressure generation systems (usually pumps and compressors) which must generate a pressure flow without ripple, since any such ripple would influence the measuring signal. Pumps or compressors generating a constant and even air flow are generally more expensive and consume more power than pressure generation systems delivering a pressure which may not be under a specific threshold. The weight or the power consumption of the unit is clearly increased.
Yet another disadvantage of the methods utilised is that such methods used to date are employed exclusively on fingers, and the finger arteries belong to the small arteries which are regulated in the flow from the body for example by the temperature of the fingers, so that the pressure in these arteries no longer corresponds to the pressure in the large arteries, in which doctors are primarily interested. For this reason the devices used hitherto (for example the Finapres marketed by Ohmeda) very clearly give the relative fluctuations in blood pressure but not in absolute values of the pressure, so that the Finapres unit was also removed from the market.
Another existing sphygmomanometer essentially uses planartonometry. An array of very small pressure receivers, which are embedded in silicon, is applied to the artery by means of compressed air bellows, whereby a computer searches out the pressure sensor outputting the clearest signal. The pressure in the bellows is no longer altered after a clear signal has been found, while the pressure curve is calibrated by one-off or multiple measuring of the oscillometric blood pressure which can be measured intermittently on the same upper arm. When a hard object is applied, namely the array onto the artery, the former deforms in an uncontrollable manner, so that the pressure values output by this unit deviate very strongly from the intra-arterial values. (Zorn et al, Blood Pressure Monitoring 2: 185, 1997). Precise analysis of the pressure curves can additionally be employed by means of an expanded Windkessel model in known fashion to evaluate compliance of the large and small vessels, as demonstrated by Waft and Burrus. Furthermore, the pressure in the central aorta can also be calculated by computer, for example with frequency analyses, or a so-called augmentation index can also be calculated which clearly reflects the actual mechanical strain on the heart and vascular system. To date the so-called aplanation tonometry, wherein a hard pressure sensor was applied by hand or per micrometer screw to the artery, has been used to relieve arterial wall. The disadvantage of this so far has been that the pressure lying on the artery because of the pressure sensor was not known, and that it was exceedingly troublesome to accurately find the artery by hand.
The object of the present invention is to prevent these known difficulties by developing a new sphygmomanometer.
According to the invention a sphygmomanometer for continuous plethysmographic measurement of blood pressure includes at least one inflatable pressure pad which is attachable to a body part containing an artery, arterial signal sensors for determining arterial blood flow, and a valve-controlled pressure chamber connected to a gas source and to the inflatable pressure pad and including a pressure sensor for measuring the pressure in the pressure chamber or in the pressure pad. The pressure chamber has separate inlet and outlet valves which are controlled dependent on signals of the arterial signal sensors.
The sphygmomanometer according to the invention is described in greater detail with reference to the attached drawings.